In the ever increasing number of electronic devices being used worldwide, electromagnetic compatibility, EMC, regulations need to be complied with to obtain approval of the devices.
Designers are facing further issues in keeping the emitted radiation below the limits due to factors like increased clock speed, coexistence of digital and analogue systems, shrinking of PCB dimensions, etc. This is especially the case with devices where the space is critical such as in hand held terminals, mobile phones or medical implants and devices. Often, the space is so limited that there is no room for traditional solutions regarding for example grounding, filtering, and shielding.
It is known in the art to provide shielding by encapsulating electronic devices in for example a metal house to avoid any radiation from the electronic devices at all. However, as more electronic devices are configured for wireless communication with other devices, external to the electronic device, this approach has its obvious drawbacks as not only noise signals are trapped but also the wireless signals could be trapped.
It is seen in the art that there sometimes is a need to attenuate a narrow frequency band or even a single frequency and its higher-order harmonics due to the nature of clock circuits, switched mode power supplies, micro wave power amplifiers and voltage-controlled oscillators, resonance phenomena in the structures, etc. which may produce unwanted radiation in a narrow frequency band.
It is known in the art to provide filtering for a transmission line, however, traditional filters typically filter a broad section of frequencies which is a disadvantage when a transmission line needs filtering for a frequency that is very close to the operating frequency for the device.
Examples of such prior art approach include electromagnetic band gap structures which have been developed in order to mitigate the interference caused by high speed digital and analogue traces on printed circuit boards. However, such structures tends to be quite large, and too large to use with small printed circuit boards. Typically, the filtering is done with lumped elements which are not practically implementable in small devices. Thus, the electrical and physical dimensions of such electromagnetic band gap structures are not suitable for applications using small printed circuit boards.
Especially for electronic devices being subject to space restrictions, such as mobile phones, medical implants, hearing instruments and hearing instrument accessories, there is a need for an improved filtering in order to mitigate the interference caused by e.g. high speed digital and analogue traces on printed circuit boards.